As Ukrainian forces close in on Russian-backed rebels in Donetsk and sanctions
threaten to bite hard in Moscow, fears are deepening of a Russian military
intervention

Russia is preparing to send troops into eastern Ukraine under the pretext of mounting a humanitarian mission to save separatist rebels, Nato and American officials have warned.

The Kremlin has doubled the number of troops on its border with Ukraine to 20,000 and is conducting a week of military exercises.

A sapper works on the site hit by an air strike in Donetsk (AFP/Getty)

Oana Lungescu, a Nato spokesman, said on Wednesday: “We’re not going to guess what’s on Russia’s mind, but we can see what Russia is doing on the ground – and that is of great concern. Russia has amassed around 20,000 combat-ready troops on Ukraine’s eastern border.”

She said Nato was concerned Moscow could use “the pretext of a humanitarian or peace-keeping mission as an excuse to send troops into eastern Ukraine”.

Chuck Hagel, the US defence secretary, said the threat of a Russian intervention in Ukraine was now “a reality” after a meeting with his senior US commanders in Stuttgart.

“When you see the build-up of Russian troops and the sophistication of those troops, the training of those troops, the heavy military equipment that’s being put along that border, of course it’s a reality, it’s a threat, it’s a possibility – absolutely,” he said.

The Nato statement came hours after Russia called an emergency meeting of the UN Security Council in New York to warn that the regions of Luhansk and Donetsk were “on the brink of a humanitarian disaster” and calling for the international community to “mobilise towards immediate assistance”.

Pressure has been building on Vladimir Putin, the Russian president, in recent days with the combination of tougher EU and US sanctions and advances made by Ukrainian government forces who are closing in on Russian-backed rebel strongholds.

Donetsk, the rebel-held city in eastern Ukraine, was hit by air strikes overnight, the first since Ukrainian forces bombarded the airport in May in their attempt to halt the insurgency in the industrial, broadly pro-Russian east of the country.

Photographs and video footage from the scene showed a 13ft crater in a road. A warehouse and an office building were reportedly hit.

There were no casualties but three civilians were killed overnight by the shelling of suburbs of Donetsk, the city council said. This included two deaths already announced on Tuesday night. Andriy Lysenko, a spokesman for Kiev's National Security and Defence Council, insisted the Ukrainian military did not bomb built-up areas.

However, the sound of jets flying over Donetsk immediately after two loud explosions was clearly audible late on Tuesday. The city’s airport is destroyed and the rebels do not have any planes.

“We heard and saw aircraft flying overhead,” said Oleg Tsarev, a separatist leader, at a briefing in a Donetsk hotel on Wednesday. “We heard the explosions and we saw the craters. The Ukrainian media and leadership claim there was no attack but we are here and see what happens. And we know that Martians did not fly in to bomb us.”

Many residents have fled the city, leaving its streets deserted even at rush hour. Several neighbourhoods are experiencing power cuts and have no water. Mr Tsarev said he thought only 450,000 of Donetsk’s million-strong population remained.

With EU and US sanctions threatening to bite hard and the Russian economy now on the brink of recession according to IMF forecasts, Mr Putin has announced retaliatory sanctions, banning selected food imports from Ukraine, Poland, Romanian and Australia.

Western diplomatic sources have told The Telegraph there are concerns that Mr Putin has become increasingly “erratic” in recent weeks.

Invasion under the pretext of a humanitarian mission was a “very real option,” a US defence department official told the New York Times on Monday. “And should Putin decide, he could do that with little or no notice. We just don’t know what he’s thinking.”

In an apparent show of solidarity, Anders Fogh Rasmussen, the Nato secretary-general, is to visit Kiev today, Ukraine’s foreign ministry said, ostensibly to discuss an upcoming meeting on the Nato-Ukraine “partnership”.

Ukraine is not a member of the 28-nation alliance and is therefore not protected by the mutual defence pact, unlike Poland and the Baltic states.

Sergei Shoigu, the Russian defence minister, visited Russian troops on peacekeeping exercises in central Russia on Wednesday and told them to “expect the unexpected”.

He told the soldiers on exercises in the Samara region: “The world has changed, and it has changed drastically. As you know from previous examples, including in this brigade, peace-keeping units can be called upon unexpectedly.”